1. Field of the Invention
The present invention relates to an optical unit for use in a laser beam printer, and more particularly to an optical unit which guides a laser beam from a laser diode to an object to be scanned, by way of a group of focusing lenses and a scanner.
2. Description of the Related Art
In general, an optical unit incorporated in a laser beam printer or the like is designed such that a laser beam output from a laser diode is guided first to a scanner, i.e., an optical deflector and then to a photo-sensitive body, i.e., an object to be scanned. The photosensitive body is scanned with the laser beam at a constant speed. Before reaching the photosensitive body, the laser beam passes through a first optical system and a second optical system. The first optical system converges the laser beam generated by the laser diode, while the second optical system focuses the laser beam to a desirable position on the surface of the photosensitive body. When passing through the first and second optical systems, the laser beam is made to have a cross section of predetermined size.
The first optical system is a combination of lenses, such as an aspheric surface glass lens, plastic lenses, etc. The second optical system is a combination of lenses, such as an f0 lens. By this f0 lens, the deflection angle at which a laser beam is deflected by the scanner is changed in proportion to the position at which the laser beam is focused on the photosensitive body and which is expressed in relation to the main scanning direction.
Published Unexamined Japanese Patent Application (PUJPA) No. 61-59311 discloses an f0 lens which is incorporated in such a second optical system as mentioned above and which has its ends supported by means of bimetal. Since the f0 lens is supported by bimetal, a variation in the focal length, which may occur due to a change in the ambient temperature, can be corrected.
In the system disclosed in the above mentioned Japanese Patent Application, the focal length of the f0 lens can be corrected in accordance with the ambient temperature. It should be noted, however, the system is not designed in consideration of adverse effects which may be caused by the ambient moisture, such as a variation in the focal length, deformation of a plastic lens, a variation in the refractive power, and a variation in the wave-length of a generated laser beam. Thus, satisfactory correction cannot be expected in the system of is Japanese Patent Application. It should be also noted that the correction using bimetal is not very reliable. Specifically, the amount of correction is not always constant, due to the thickness of the bimetal. In addition, since the lens itself is moved for correction, its optical axis may tilt, resulting in geometric distortion in the entire focusing system.